Outlook mails are duplicated in Inbox, Mail server

[James Robert 0]

Hi,

since a week ago, Mail Server is not detecting duplicate emails. After testing sending emails from www.outlook.com, I see 5+ duplicates per hour in my Synology inbox. In the mail log, I see that these emails do not have a Message there.
Nevertheless, when examining these messages in Mail Station or the Outlook mail app, message IDs are present for each of them. Every duplicate has an identical message ID.
The duplicates clutter up the inbox and keep coming in for about 72 hours, which is really annoying.
As far as I can see, there is only a problem with emails from www.outlook.com, but there might be others.

Has someone seen this and figured out how to fix it?

Thanks!

[Marcos 5,082]

I only assumes the issue concerns Microsoft Exchange with ESET Mail Security installed or another product. Please raise a support ticket.